Most blood analyzer systems in use today measure microscopic particles/cells in a blood sample by means of electrically and/or optically based measurements. In a Coulter/impedance-based electrical measurement system, particles within a carrier fluid passing through a detection aperture cause the generation of electrical pulses proportional to their volume; using some form of thresholding technique on the pulses, the particles that give rise to the pulses may be classified as platelets, erythrocytes (red blood cells (RBCs)), or leukocytes (white blood cells). As shown in FIG. 1, in an ideal case, the trajectory of a particle 10 passing through the detection aperture 12 will be coincident with its central axis 14, so as to produce a pulse as shown at 200 in FIG. 2, the pulse having a central peak 201 that is bounded by a pair of relatively symmetrically rising and falling sloped portions 202 and 203. In the real world, however, many pulses travel through the detection aperture along non-axial trajectories, as shown at 11 in FIG. 1. Because the electrical field strength is non-uniform throughout the aperture, a particle passing near an edge of the aperture will typically give rise to what is classically referred to as an ‘M’ shaped pulse, shown at 300 as having a pair of peaks, as shown at 301 and 302 in FIG. 2 on either side of the center 303 of the pulse. Thus, the peak value for identical particles will vary depending upon the path of the particles through the aperture.
One very successful prior art technique developed by the assignee of the present application, and described in the U.S. patent to Doty et al, U.S. Pat. No. 3,710,263, involves an editing technique that discards undesirable pulses based upon their width at 50% and 75% of the peak. Editing, however, has the disadvantage that it loses a captured event, thereby requiring more time to build up an accurate histogram of the particle size distribution. In addition, this technique applies the same pulse width limits on all pulse heights, potentially over-editing some populations and under-editing others. An alternate sizing scheme, using the pulse amplitude at the time the particle is halfway through the aperture, has long been considered a superior technique for more accuracy without editing, yielding an accurate result more quickly with fewer pulses. The U.S. patent to Doty, U.S. Pat. No. 3,863,160, describes an analog system for accomplishing this approach. This analog system uses integration and delay lines to find a mid-flight point which, unfortunately, is inaccurate on asymmetric pulses, has limited throughput due to fixed delays and circuit state overhead, and is difficult to construct due to tolerance stack-up.